12978
J. Phys. Chem. 1994, 98, 12978-12988
Cluster Catalysis: Propane Hydrogenolysis Catalyzed by MgO-Supported Tetrairidium S. Kawi, J.-R. Chang, and B. C. Gates* Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware I971 6, and Department of Chemical Engineering and Materials Science, University of Califomia, Davis, Califomia 95616 Received: February 1, 1994; In Final Form: October 4, 1994@
A metal carbonyl cluster with a tetrahedral metal frame, [Hk4(CO)11]-, was prepared on the surface of partially hydroxylated MgO powder by adsorption of [Ir4(CO)12]. A supported catalyst was prepared by decarbonylation of the supported [Hzr4(CO)11]- by treatment in He followed by Hz at 300 "C. The surface structures were characterized by inf'rared and X-ray absorption spectroscopies. The decarbonylated cluster had an average Ir-Ir coordination number of 3.4, consistent with the hypothesis that the tetrahedral metal frame was largely retained after decarbonylation. The sample incorporating the decarbonylated clusters was catalytically active for propane hydrogenolysis at 1 atm and 200 "C. The catalyst that had been used for 1 day in a flow reactor was characterized by an Ir-Ir coordination number of 3.3. The lack of any significant change in the average cluster size indicates the stability of the supported catalyst, which is modeled as tetrahedral Ir4 clusters. The rate of the propane hydrogenolysis at 200 "C and atmospheric pressure with a H2/propane molar ratio of 2.5 was 0.02 molecule of CHq produced(Ir4 cluster s).
Introduction The prospect of selective catalysis by molecular metal clusters has tantalized researchers since Muetterties' essay suggesting that metal clusters would be novel catalysts because they have unique structures that allow bonding of reactants in ways that mimic bonding at metal surfaces.2 However, the prospects of novel cluster catalysis remain largely unrealized, and the stability limitations of molecular metal clusters lead one to be less than optimistic about the prospects today. However, the common form of metals applied in industrial catalysis is highly dispersed crystallites or aggregates that are often small enough to be called cluster^.^ For example, catalysts used in the refonning of naphtha consist of clusters of Pt and Re dispersed on the surface of an amorphous support (carrier) such as y-Al~O3.~ In a new commercial process that involves similar chemistry and is characterized by an extraordinarily high selectivity, naphtha undergoes dehydrocyclization to give aromatic^^-^ catalyzed by clusters of, on average, 5 or 6 Pt atoms in the channels of a crystalline support, zeolite LTL.l0J1 In contrast to the metal carbonyl clusters tried most frequently as catalysts in solution,12 the supported clusters are stable, maintaining their activity and selectivity for long periods of continuous operation. Thus, there is still some justification for Muetterties'' optimistic forecast.13 But the practical approach of using metal cluster catalysts dispersed on supports such as metal oxides or zeolites carries with it an imposing challenge in the characterization of the chemistry because of the smallness of the catalytic species and their presence in a dispersed state in solids. Only recently have physical methods for identification of small dispersed clusters become available, including, for example, extended X-ray absorption fine structure (EXAFS) spectroscopy,14high-resolution transmission electron microscopy (HRTEM),15and scanning tunneling microscopy (STM) and atomic force microscopy (AFM). HRTEM is difficult for clusters as small as those containing only a few atoms of metal, because the samples are air-sensitive and because they may be damaged
* To whom correspondence should be addressed at the University of California. @Abstractpublished in Advance ACS Absrracrs, November 15, 1994.
by the electron beam; STM and AFM have not yet been applied to well-defined supported clusters. Our goals in the research described here were to prepare small, nearly uniform metal clusters on a metal oxide support and to determine their catalytic properties for a class of reaction that takes place in the reforming of hydrocarbons, namely, alkane hydrogenolysis. EXAFS spectroscopy was used to characterize the supported metal clusters before and after catalysis of propane hydrogenolysis in a continuous flow reactor. MgO-supported iridium clusters were prepared from [Ir4(CO)12] to give supported catalysts with nearly unique structures, represented as I r a g O . 1 6 9 1 7 A conventional preparation with aqueous H2IrCl6 was also used to prepare a more typical supported metal catalyst that consisted of crystallites of Ir of various sizes and shapes on the MgO support.
Results Characterizationof Catalysts by Transmission Electron Microscopy. Transmission electron micrographs showing Ir clusters produced by reduction in H2 of the sample formed from H2IrCl6 and MgO at 425 "C are nonuniform and about 50-100 A in diameter; they indicate that there had been significant agglomeration of the Ir. The very small supported Ir clusters formed from [Ir4(CO)12] were not characterized by this technique. Characterization of Surface and Extracted Species by Infrared Spectroscopy. The catalysts were prepared by adsorption of [Ir4(CO)12] on MgO powder followed by treatment to remove the CO ligands. The light yellow solid that was initially formed is characterized by an infrared spectrum (Figure 1B) different from that of [Ir4(CO)12] in THF (Figure 1A) and closely resembling the spectrum of [HIr4(C0)111- (Figure lC, Table 1). The adsorbed species could not be extracted with neutral solvents such as methanol, but extraction with bis(triphenylphosphine)nitrogen(1+) chloride, IppN][Cl], in methanol was efficient, consistent with the occurrence of cation metathesis, which implies the presence of ionic species on the MgO surface. The yellow extract solution is characterized by an infrared spectrum matching that of [ H I ~ ~ ( C O ) ~ I ] - The .'~J~
0022-3654/94/2098-12978$04.50/0 0 1994 American Chemical Society
J. Phys. Chem., Vol. 98, No. 49, 1994 12979
Cluster Catalysis
Figure 1. Infrared spectra of [Ir4(CO)l2] and the sample prepared by adsorption of F4(CO)12] on MgO: (A) [Ir4(C0)12]in THF,(B) spectrum of sample prepared by adsorption of [Ir4(C0)1~]on MgO; (C) solution spectrum of the species extracted from MgO with [PPN][Cl] in methanol.
solid after extraction was again white, the color of MgO. The anion was the only organometallic species detected in the extract solution. We infer that [Ir4(CO)12] had been chemisorbed on MgO by reacting with the surface OH groups to give [HIr4(CO)11]-. This inference is in agreement with the results of earlier experimentsl6S2Oin which the same surface species was identified by infrared and EXAFS spectroscopies and by extraction into solution and identification by infrared spectrosCOPY The infrared fingerprint in the vco region of the surface species (Figure 1B) is similar to that of [HIr4(CO)11]- in methanol solution (Figure lC), except that (1) the terminal carbonyl bands are shifted about 27 cm-l to higher frequencies, and the bridging carbonyl band is shifted about 44 cm-I to lower frequency, and (2) the peaks are broadened, as is typical of the spectra of metal carbonylates on nonuniform surfaces of metal oxides. The comparison suggests that [HIr4(C0)11]- was present in ion pairs on the MgO surface.20*21The suggestion of other peaks in the spectrum of Figure 1B may be an indication that metal carbonyl species other than [HIr4(CO)11]- were also present on the surface, but the spectra are not sufficient to identify them. EXAFS Data Characterizing Sample Prepared by Decarbonylation of [HIr4(CO)11]- on MgO. The sample incorporating [HIr4(CO)11]- on MgO was decarbonylated by a treatment similar to that Infrared spectra recorded during the decarbonylation at 300 OC showed that the vco peaks had disappeared within about 1 h, consistent with the complete removal of the CO ligands. EXAFS data from two or three scans characterizing the decarbonylated sample were averaged. The normalized EXAFS 9
function was obtained from the averaged X-ray absorption spectra by a cubic spline background subtraction and normalized to the edge height.22323The EXAFS (chi) function characterizing the sample is shown in Figure 2A. The data show reliable oscillations up to a value of k = 14 A-1 (k is the wave vector). Oscillations in the intermediate and higher ranges of k (8 < k < 14 A-1) are indicative of Ir-Ir interactions. The E M S analysis was done with experimentally determined reference files, described below. The analysis of the EXAFS data was performed with the Vaarkamp-LindersKoningsberger XDAP software, with the goodness of fits evaluated by a least-squares criterion. In the initial stage, the analysis was done on the isolated part of the spectrum corresponding to the first coordination shell of the Ir atoms. The EXAFS data were Fourier transformed with k2 weighting and no phase correction over the useful range (3.09 < k < 14.07 A-1). The major Contributions were isolated by inverse Fourier transformation in the range 0.76 < r < 3.32 A, where r is the distance from the absorber atom. With the Koningsberger difference file t e ~ h n i q u e ?the ~ , ~Ir-Ir ~ contribution, the largest in the EXAFS spectrum, was estimated by calculating an EXAFS function that agreed as closely as possible with the experimental results in the range 7.50 < k < 13.50 A-1; the metal-support contributions affect the data negligibly in this region because the backscatterers present on the support surface have such low atomic weights. An EXAFS function calculated with the first-guess parameters was then subtracted from the data, with the residual spectrum being expected to represent the Ir-Osupportinteractions (where Osupport represents 0 on the MgO surface). The difference file was estimated with two Ir-0 contributions, since both short26-28 and long25$29*30 metalsupport oxygen distances have been frequently observed.31As a first approximation, only four free parameters were estimated. (Au2, the Debye-Waller factor, and AEo, the inner potential correction, were set equal to 0 for each contribution to shorten the computational time.) The first-guess Ir-Ir and Ir-Osupportcontributions were then added and compared with the raw data in r space, and the fit was not satisfactory. Then the estimated Ir-Osupponcontribution was subtracted from the data, and better parameters for the IrIr contribution were estimated. The improved fit for the Ir-Ir contribution was subtracted from the data, and more accurate parameters for the contributions of the metal-support interface were determined by fitting the metal-support contributions to the residual spectrum with all eight parameters; the initial guesses for parameter estimation were determined by adjusting the coordination parameters to give the best agreement with the residual spectrum, both in k space and in r space, with both k1 and # weighting. The iteration was continued until the fit was good. The Ir-lr and two Ir-OSumrt contributions were then added, representing the overall fit of the data. To show the goodness of the fit for both the high-Z (Ir-Ir) and low-Z (Ir-0) contributions, the raw data are compared with the fit, both in k space (with k2 weighting) and in r space (with both k' and k3 weighting) (Figure 2B-D). The agreement is good.
TABLE 1: Infrared Spectra in the Carbonyl Stretching Region of the MgO-Supported Species Formed from Adsorption of rIr4(co)121 sample [Ir4(CO)121in THF sample prepared by adsorption of [Irh(C0)12]on MgO extract of above sample with [PPN][Cl] in methanol ~t41[HIr4(CO)111in TI-F Na[HIr4(C0)11]in diethyl ether
VCO,
cm-'
2040 s, 1955 s 2073 w, 2044 s, 2008 m, 1985 w, 1768 vw 2030 sh, 2017 vs, 1990 m, 1980 sh, 1812 w 2067 w, 2030 s, 2017 vs, 1986 m, 1978 m, 1832 m, 1806 m 2072 w, 2039 s, 2020 vs, 1990 m, 1984 m, 1730 m, 1830 w
ref this work this work this work 17 17
Kawi et al.
12980 J. Phys. Chem., Vol. 98, No. 49, 1994 1
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r (A> Figure 2. Results of EXAFS analysis obtained with the best calculated coordination parameters for the supported Ir catalyst prepared by the decarbonylation of the iridium carbonyl clusters on MgO at 300 "C, in flowing He followed by flowing Hz: (A) raw EXAFS data; (B) k2-weighted Ir-Osum contributions (dashed line); (C) imaginary part and magnitude of experimental EXAFS (solid line) and sum of the calculated Ir-Ir (Ir-O1+ Fourier transform @'-weighted, Ak = 3.50-13.50 A) of experimental EXAFS (solid line) and sum of the calculated Ir-Ir Ir-Osupport Ir-Os) contributions (dashed line); (D) imaginary part and magnitude of Fourier transform (k3-weighted;Ak = 3.50-13.50 A) of experimental E M S (solid line) and sum of the calculated Ir-Ir Ir-Osuw contributions (dashed line); (E) residual spect" illustratingthe EXAFS contributions characterizing the metal-support interaction: imaginary part and magnitude of Fourier transform (kl-weighted,Pt-0 phase-corrected, Ak = 3.5011.50 A-') of raw data minus calculated Ir-Ir EXAFS (solid line) and calculated Ir-01 Ir-Os EXAFS (dashed line).
+
+
+
+
The structure parameters determined by the fit are summarized in Table 2, and the Ir-Osupponcontributions are shown in the difference file of Figure 2E. The number of parameters used to fit the data in this main-shell analysis is 12; the statistically justified number n is approximately 17, estimated from the Nyquist theorem,14 n = (2AkAr-h) 1, where Ak and Ar respectively are the k and r ranges used in the forward and inverse Fourier transforms (Ak = 10 A-1; Ar = 2.56 A). An estimate of error bounds on the parameters was made with the XDAP software by using the signal to noise ratio at k equal
+
to about 14 A-1 (Table 2); the error bounds are approximations of the standard deviations. A similarly prepared sample was characterized by EXAFS spectroscopy by van Zon et al.," whose results characterizing the Ir-Ir contribution are nearly the same as ours. However, van Zon's results characterizing the metal-support interface are different from ours; in particular, they identified a weak contribution that they suggested was Ir-Mg, although they stated that this is not well understood. Because the additional contribution is small and not well understood, we consider the
Cluster Catalysis
J. Phys. Chem., Vol. 98, No. 49, I994 12981
TABLE 2: EXAFS Results Characterizing the Iridium Clusters Formed by Decarbonylation of [Ir4(C0)12]on MgO at 300 "C in He Followed by Hf Aa2 x 1O00, A2 AEo,eV EXAFS ref shell N R, Ir-Ir 3.35 f 0.15 2.730 f0.005 3.62 f0.56 -5.89 f0.37 Pt-Pt ~r-os"ppon Ir-Os Ir-01 a
0.86 f 0.26 1.13 f 0.24
2.176 f 0.055 2.691 f 0.015
9.42 f 5.34 -5.18 f 1.51
-15.41 f4.33 -5.71 f 1.52
Pt-0 Pt-0
N is coordination number, R the average absorber-backscatterer distance, Aa2 the Debye-Waller factor, and AEo the inner potential correction.
The subscripts 1 and s refer to long and short, respectively.
Methane
Ethane
Hydrocarbon Products Figure 3. Comparison of the activities of MgO-supported Ir catalysts, one prepared from [Ir4(CO)12] and the other prepared conventionally from H21rC16. The reaction was carried out at 200 "C and 1 atm in a once-through flow reactor. The partial pressure of H2 was 0.25 atm, and the partial pressure of propane was 0.75 atm.
analysis that does not account for it to be more appropriate. The differences in the details of the structure of the metalsupport interface in van Zon's work and in ours are unresolved; they may have to do with the relatively long time between preparation and EXAFS measurements with van Zon's sample and the possibility that some impurities affected their sample slightly. Kinetics of Catalytic Propane Hydrogenolysis. The two MgO-supported Ir samples, one prepared from [Ir4(CO)12] and the other prepared conventionally from H2IrCl6 and incorporating nonuniform Ir crystallite^,^^ were used to investigate the effect of Ir cluster (crystallite) size on catalytic performance for propane hydrogenolysis. The experiments were carried out with a conventional once-through flow reactor interfaced to a gas chromatograph. Separately, in experiments described below, the catalytic reaction was carried out at the same temperature and pressure in an EXAFS cell. A temperature of about 200 "C was the minimum required for convenient measurement of the activity of the catalyst derived from [Ir4(CO)12]. The conventional catalyst was more active, having a measurable activity at about 160 "C. The reaction products were methane and ethane, with slightly greater yields of the former than the latter, indicating a some secondary reaction of the ethane. The activities of the two catalysts were measured at 200 "C. The conversion to methane was observed to be proportional to the inverse space velocity, confirming that the conversions were differential and determining reaction rates directly. The reaction rate at 200 "C with a Hdpropane molar ratio of 2.5 was 2.6 x [mol of methane produced/(g of Ir s)] for the catalyst derived from [Ir4(CO)12] and 3.2 x in the same units for the catalyst prepared from the iridium salt (Figure 3). Thus, the former catalyst is 2 orders of magnitude less active than the conventional catalyst for propane hydrogenolysis. Data showing the catalyst stabilities during the propane hydrogenolysis reaction in the flow reactor are given in Figure
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200
300
400
500
600
Time on Stream, min
Figure 4. Stability of MgO-supported Ir catalysts: (A) catalyst prepared from [Ira(CO)12]; (B) catalyst prepared conventionally from H2IrCl6. The conditions were the same as those stated in the caption of Figure 3. 4A,B. Both supported iridium catalysts lost activity during the catalytic reactions. The catalyst derived from [Ir4(CO)12] attained a virtual steady state in a shorter time than the conventional catalyst. The kinetics of propane hydrogenolysis was measured for each catalyst in a virtual steady state (following the initial deactivation) as the feed composition was varied systematically. In all the experiments reported here, propane conversions were differential (between 1% and 5%). The dependence of the rate of propane hydrogenolysis on propane partial pressure is shown for each catalyst in Figure 5. Similar plots show the dependence of rate on hydrogen partial pressure (Figure 6). The two catalysts are characterized by similar dependences of the rate on the reactant partial pressures at low reactant partial pressures, but the patterns are different at the higher partial pressures. EXAFS Data Characterizingthe Used IIr~(CO)12]-Derived Catalyst after Propane Hydrogenolysis. The sample that had been used as a catalyst for propane hydrogenolysis in the EXAFS cell was characterized by EXAFS spectroscopy. The EXAFS function is shown in Figure 7A. The data show reliable oscillations up to a value of k = 14 A-1. Again, oscillations in the intermediate and higher ranges of k (8 < k < 14 A-l) are indicative of Ir-Ir interactions. The data are nearly the same as those characterizing the sample before use as a catalyst. The method of E M S data analysis was virtually the same as that stated above. The data were Fourier transformed over
Kawi et al.
12982 J. Phys. Chem., Vol. 98, No. 49, 1994 2
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0
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h p a n e Partial Pressure, atm
An EXAFS function calculated with the first-guess parameters was then subtracted from the data, with the residual spectrum being expected to represent the Ir-Osumfi interactions. The difference file was estimated with two Ir-0contributions. The Ir-Osuppoficontributions were then subtracted from the data, and better parameters for the Ir-Ir contribution were estimated. The improved fit characterizing the Ir-Ir contribution was subtracted from the data, and more accurate parameters were determined by fitting the Ir-Osupponcontributions to the residual spectrum. After many iterations, the fit was still not good in the low-r region. It was thus inferred that another small contribution, attributed to a low-Z backscatterer, had to be accounted for. A difference file was calculated by subtracting the best estimated Ir-Ir Ir-Osupportcontribution from the experimental EXAFS function. The additional contribution was calculated by fitting the difference file with four adjustable parameters. The additional low-Z scatterer is not identified; it is likely to be carbon on the catalyst surface that was formed during the propane hydrogen~lysis,~~ and we refer to the contribution as Ir-C. The Ir-Ir, Ir-C, and two Ir-Osuppfi contributions were then added, representing the overall fit of the data. To show the goodness of the fit for both the high-Z (Ir-Ir) and low-Z (Ir0, Ir-C)contributions, the raw data are compared with the fit, both in k space (with k2 weighting) and in r space (with both k1 and k3 weighting) (Figure 7B-D).34 The agreement is good. The structure parameters are shown in Table 3, and the IrOsuppon contributions are shown in the difference file of Figure 7E. The number of parameters used to fit the data in this mainshell analysis is 16; the statisticallyjustified number, calculated as above, is approximately 17. X-ray Absorption Near Edge Structure (XANES). Ir Lm absorption edge data are shown in Figure 8 for the supported Ir sample formed by decarbonylation of [HIr4(C0)11]- on the MgO support and for the used catalyst. The absorption edges characterizing the two samples are normalized to the height of the absorption edge determined at 50 eV past the edge position. The white lines characterizing the two samples are very similar to each other, indicating that the average oxidation states of the Ir atoms in the two samples are almost the same.
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Figure 5. Dependence of the rate of propane hydrogenolysis on propane partial pressure: (A) catalyst prepared from [Ird(C0)12]; (B) catalyst prepared conventionally from HzIrC4. The H2 partial pressure was fixed at 0.10 atm. 3 L .
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Figure 6. Effect of HZpartial pressure on the activities of the MgOsupported catalysts: (A) catalyst prepared from [Ir4(CO)12]; (B) catalyst prepared conventionally from HzIrCk. The propane partial pressure was fixed at 0.10 atm.
the useful range (3.00 < k < 14.63 A-l) with k2 weighting and no phase correction. The major contributions were then isolated by inverse Fourier transformation in the range 0.76 < r < 3.30 A. The Ir-Ir contribution was estimated first by calculating an EXAFS function that agreed as closely as possible with the experimental results in the high-k range (7.50 < k < 13.50 A-1).
Discussion Synthesis of [€IIr4(C0)11]- on the MgO Surface. The results are consistent with the conclusion that [Ir4(CO)121 adsorbed on the MgO surface was initially converted into supported [HIr4(CO)ll]-.16~20The extraction of this anion into solution is consistent with the inference that it was present on the surface, and the color and infrared spectra of the solid are consistent with the inference that this anion was the predominant organometallic species on the surface. The infrared spectrum of the surface species is similar to that of [HIr4(C0)11]- in THF solution, except for the shift of the terminal carbonyl bands to higher frequencies and that of the bridging carbonyl band to lower frequency. However, the peaks characterizing the surface species are broad, consistent with the nonunifomity of the MgO powder surface and the interaction of [HIr4(C0)11]- with various surface sites. Because of the breadth of the bands, we cannot rule out the possibility that some other iridium carbonyl species were also present. The shifts in the bands attributed to [HIr4(CO)~1]-relative to those of this anion in methanol solution are inferred to result from the interaction of the bridging carbonyl ligands of the anionic cluster with surface Mg2+ ions (Lewis acid sites); the effect is analogous to those observed for numerous metal carbonyl clusters present in solutions with Lewis a ~ i d s .For ~~,~~
Cluster Catalysis 0.04
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J. Phys. Chem., Vol. 98, No. 49, 1994 12983
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r (A) Figure 7. Results of EXAFS analysis obtained with the best calculated coordination parameters for the supported Ir catalyst prepared from the decarbonylation of the iridium carbonyl clusters on MgO at 300 "C, in He and Hz, followed by propane hydrogenolysis at 200 "C in an EXAFS cell: (A) raw EXAFS data; (B) k2-weighted experimental EXAFS (solid line) and sum of the calculated Ir-Ir Ir-C Ir-Osupw contributions (dashed line); (C) imaginary part and magnitude of Fourier transform (kl-weighted, Ak = 3.50-13.50 A) of experimental EXAFS (solid line) and Ir-C Ir-Osuppn (Ir-01 Ir-Os) contributions (dashed line); (D) imaginary part and magnitude of Fourier sum of the calculated Ir-Ir transform @-weighted, Ak = 3.50-13.50 A) of experimental EXAFS (solid line) and sum of the calculated Ir-Ir Ir-C Ir-Osuwn contributions (dashed line); (E) residual spectrum illustrating the EXAFS contributions characterizing the metal-support interaction: imaginary part and magnitude of Fourier transform (kl-weighted, pt-0 phase-corrected, Ak = 3.50- 11S O of raw data minus calculated Ir-Ir Ir-C EXAFS (solid line) and calculated Ir-01 Ir-Os EXAFS (dashed line).
+
+
+
+
example, Vandenberg et al.*' observed that [HIr4(CO)111- is ion paired with Na+ in diethyl ether solution, resulting in a shift of the major terminal carbonyl band of about 17 cm-' to higher energies and a shift of the bridging carbonyl band of about 100 cm-l to lower energy. Advantages of Organometallic Precursors in Preparation of Structurally Simple Supported Metal Catalysts. The cluster-containing samples that are expected to give the most precise structural information from EXAFS spectroscopy are
+
+
+
+
+
those that contain the most nearly uniform clusters. A unique advantage of the organometallic precursors is the opportunity they offer for preparation of extremely s m d and nearly uniform supported metal clusters that are well suited to characterization with EXAFS spectroscopy. The results presented here give evidence that the tetrairidium clusters can be decarbonylated on the support with little change in nuclekty (Table 2). Thus, an important result of the EXAFS analysis for the sample prepared from [Ir4(C0)12] on MgO is the evidence that the
12984 J. Phys. Chem., Vol. 98, No. 49, 1994
Kawi et al.
TABLE 3: EXAFS Results Characterizing the Catalyst after Use for Propane Hydrogenolys* shell
N
R, A
EXAFS ref
3.26 f 0.23
2.735 f 0.006
A d x 1000, A2 3.87 f 0.68
AEo, eV
Ir-Ir Ir-Osuppart Ir-Os Ir-0, Ir-C
-1.91 f 0.57
Pt-Pt
1.32 f 0.24 1.87 f 0.30 1.93 f 0.18
2.209 f 0.025 2.750 f 0.019 1.893 f 0.011
5.64 f 3.36 0.50 f 2.23 9.46 f 2.26
-10.68 f 2.78 -10.50 f 1.92 -8.94 f 0.95
Pt-0 Pt-0 Ir-C
Notation as in Table 2. 2
n
